In many radio frequency (RF) technology applications, a voltage supply, providing power to a transmitter, has some level of noise on it. For example, when a transmitter is installed in an automotive vehicle, such as an automobile, an alternator in the automobile generates alternator whine noise that shows up on the voltage supply to the transmitter.
Due to the sensitivity of a RF power amplifier (PA) in the transmitter, especially in direct frequency modulation (FM) transmitters, noise on the RF PA causes amplitude modulation (AM) on the PA. This voltage supply fluctuation can be converted to FM by several paths such as direct AM to phase modulation (PM) conversion in the PA or by impedance reflections back to the PA (otherwise known as “load pull”) causing a voltage controlled oscillator (VCO) in the transmitter to FM the noise. If the frequency-modulated noise is in baseband frequencies, including audio frequencies, the frequency-modulated noise will be on a signal transmitted by the transmitter to a corresponding receiver. The receiver will demodulate the frequency modulated noise on the signal to thereby degrade the received baseband signal. Isolating the PA from the voltage supply in the transmitter typically solves this problem.
Automobiles commonly use a large inductor-capacitor (LC) filter to reduce noise generated by the alternator. But to handle the current, frequency, and amplitude of the noise, an inductor of several cubic inches that is quite heavy is needed. Furthermore, there is a resistive drop in the inductor, causing a voltage drop to the PA and therefore putting tighter design constraints on the power output required at a given voltage level.
Automobiles also commonly regulate a product's supply voltage at a fixed voltage below an available voltage supply. However, when a battery is used for the available voltage supply, the fixed voltage drop needed to isolate the noise becomes a large part of the available voltage supply as the battery voltage lowers near the end of the battery's life. This too puts a large constraint on the PA design, requiring larger PA devices and higher current drain since the PA is required to operate at voltage supply levels lower than the available voltage supply. If the PA is not designed for the lower supply voltage level, the power output specifications of the product may not be met at the lower voltage levels.
Automotive vehicles, such as an automobile, sometimes use an analog 3W cellular transceiver as a telematics network access device (NAD). In some installations, the car's direct current (DC) battery voltage supply (B+) provides the voltage supply directly to the RF PA. Due to voltage limitations of the PA devices when the voltage supply from the battery rises above 18.5V, a voltage clamp circuit typically is used to limit the battery voltage supply to the PA to about 18V. With the voltage clamping circuit, the PA supply voltage is generally about 200 mV below the battery voltage supply, so it essentially follows the battery voltage supply. In a typical automobile, 18V is usually seen with excessive alternator whine on a “high line” battery voltage supply (i.e., 16V), or with load dump, or with dual battery voltage supplies (24V), as used with some commercial automobile starting systems. However, when alternator whine tests are performed on a cellular transceiver NAD, alternator whine noise in the form of hum and noise (H&N) is detected due to either AM to PM conversion, load pull on the VCO from PA impedance changes with the whine variation on the supply, or some other means. Hence, providing the PA with a supply voltage having much less noise would significantly improve the H&N performance during the alternator whine test.
Two alternatives are conventionally used to remove the alternator whine in a cellular transceiver NAD. First, a fast voltage regulator that is fast enough to remove voltage spikes and that is regulated at 10V may be used to provide the whine performance needed at “low line” battery supply voltages (i.e., 10V to 12V). However, a fast voltage regulator causes two problems: presently deployed PA's cannot produce enough power at 10V without being redesigned and the power dissipation in such a pass device at 16V would be about 6W thereby requiring excessive heat sinking and a very large pass device.
Second, a tracking regulator may be used to track the battery supply voltage at a fixed voltage below the battery supply voltage. The tracking regulator can also be designed using an adjustable regulator integrated circuit (IC) and can minimize the power dissipation problem at high voltages. To effectively remove alternator whine from the supply requires about 0.8V to 1V drop from the battery supply voltage when there are 1V whine humps and an even greater drop with higher whine humps. However, at lower battery supply voltages (10-12V), a voltage drop of greater than 1V could still not meet the Electronic Industry Association's (EIA) transmit power output requirements for a transmitter at a power step zero (i.e., the highest transmit power level).
The following U.S. patents disclose improvements or implementations for voltage regulators: U.S. Pat. Nos. 5,926,384; 5,828,620; 5,815,445; 5,703,470; 5,699,313; 5,635,816; 5,422,599; 5,406,523; 5,359,552; 5,353,215; 5,268,871; 5,267,201; 5,247,581; 5,903,612; 4,843,285; 4,811,236; 4,733,140; 4,656,553; 4,644,251; 4,378,530; 4,204,147; 4,189,670; 4,025,841; 3,967,312; 5,929,619; 5,642,033; 4,958,119; 4,791,349; 4,490,779; and 4,081,740. However, these patents do not teach or suggest, alone or in combination, using a voltage regulator to remove alternator whine.
Accordingly, there is a need for a voltage regulator for a cellular transceiver NAD that optimizes conflicting requirements of the EIA's transmit power output level and the requirement for the voltage drop between the battery supply voltage and the regulator output voltage to create the needed noise immunity, while giving operational priority to the EIA's transmit power output level.